How lightning shoots for the stars

Duke University researchers have explained the dynamics of gigantic, spaceward propagating jets of lightning, such as the one seen here near the university. The jets can reach 50 miles high.

On rare occasions, jets of lightning escape from the tops of thunderclouds and shoot up into the atmosphere where they pose a threat to weather balloons and other scientific instruments. New research explains how it happens.

"In some instances there is enough energy and electric charge available for that lightning to just keep propagating up and up and up and it keeps going to about 50 miles high," Steven Cummer, a lightning expert at Duke University, told me today.

The jets come to a halt at 50 miles high because they run into the ionosphere, the electrically conducting part of the atmosphere, which "sort of shorts it out and prevents it from getting any farther," he added.

Cummer is a co-author of a paper accepted May 5 for publication in Geophysical Research Letters that, for the first time, explains the dynamics that lead to the gigantic jets of lightning shooting from the tops of clouds into the upper atmosphere.

Bolt from the blueThe jets, according to the research, appear to be related to a phenomenon called a bolt from the blue. This occurs when a lightning channel develops inside a cloud but is unable to find enough electric charge of the opposite sign to make it stop, so it shoots out the side of the thundercloud.

Sometimes when this happens, the channel will propagate a few handfuls of miles horizontally before it turns down and becomes a cloud-to-ground lightning stroke.

"That is called a bolt from the blue because if you were on the ground, you would be at best dimly aware that there was a thunderstorm 10 miles away and yet a lightning stroke came down and struck the ground whereas the sky above looks clear and blue," Cummer explained.

What appears to happen to allow the spaceward propagating jets is that a bolt from the blue starts to develop but doesn’t quite make it out of the side of the cloud. Then, a fraction of a second later another channel develops upward and escapes from the top of the cloud.

The first, failed bolt from the blue, Cummer explained, appears to deplete the upper cloud layer of any opposite charge that would normally stop the lightning stroke, opening a channel for the second part of the flash. It escapes from the cloud and turns into a gigantic jet that "just keeps going and keeps going."

Cool, rare phenomenonThese giant, spaceward propagating jets of lightning were first observed about 10 years ago. Scientists think they are rather rare, though they are more difficult to observe than cloud-to-ground lightning because it's hard to see above thunderstorms.

Researchers have viewed them a couple hundred miles away from the storms, with clear skies above and a view of the storm on the horizon. They have also been seen with cameras on satellites. From these counts, they are certainly less common than cloud-to-ground lightning, Cummer noted.

However, they are common enough to pose a slight risk to weather balloons and other scientific instruments that spend time above the clouds but below the ionosphere. "You do have to be aware that there is a non-zero chance that a lightning bolt can shoot out of a thundercloud," he said.

But studying the jets, Cummer admits, is mostly just scientifically interesting. Besides that, they have an undeniable "cool factor," he said.